This application is based on and claims priority from Japanese Application No. HEI 4-16901 filed Jan. 31, 1992, the disclosure of which is incorporated herein by reference.
This invention relates to a reading lens system for use with a facsimile apparatus, an image scanner or other optical equipment.
Reading lens systems of the type contemplated by the present invention must ensure a resolution comparable to that achieved by a CCD sensor or other image-receiving device to be used. To this end, a resolution of about 7 .mu.m is necessary for the image part. To attain a satisfactory contrast at such a high spatial frequency and to minimize the potential drop in the brightness at the edge of the image field, the aperture efficiency must be 100% while the distortion is effectively corrected.
To meet these conditions, certain Gauss-type lenses have heretofore been used. See, for example, Japanese Patent Laid-Open Publications No. SHO 50-62631, No. SHO 59-90810, etc.
However, such conventional Gauss-type lenses have had the problem that when they have an F number of 1:4 and are used at a half-view angle of about 20 degrees, they have a substantially different performance in the sagittal and meridional directions. Particularly, the lens performance in the sagittal direction has not been sufficiently considered.
This problem may be explained as follows. The CCD sensor conventionally used as an image-receiving device in facsimile and other optical equipment is a line sensor having pixels arranged on a single line. Hence, as long as the performance of a reading lens is satisfactory in the meridional direction parallel to the alignment of pixels, no deleterious effect occurs in the reading operation even if the lens performance is somewhat poor in the sagittal direction.
The recent advances in color reproduction technology have been remarkable in facsimile and image scanners, and an apparatus has been introduced that uses a CCD sensor having pixels arranged in three lines and which can read an image in one scan cycle after the image has been color-separated with a wavelength-dependent filter provided in each pixel line.
The conventional Gauss-type lenses have the problem that if they are used with such a color reproduction apparatus, the lenses cannot achieve a faithful color reproduction because of poor lens performance in the sagittal direction. Furthermore, most of the conventional reading lenses are designed for monochromatic reproduction and occasionally experience substantial variations in aberrations at a specific wavelength, which makes these lenses unsuitable for color reproduction.